This invention relates to analog-to-digital converters and, more particularly, to an analog-to-digital converter having a large dynamic input range and a linear digital output.
An analog-to-digital converter (A/D) is commonly utilized when it is necessary to represent in digital form the magnitude of an analog signal. This is typically the case when it is desired that a digital computer be capable of determining the magnitude of an analog signal, as is often done in process-control applications. For example, if a certain process were to be controlled, and a key variable parameter of the process were a temperature, a thermocouple might be utilized to measure the temperature. The analog voltage appearing at the output of the thermocouple would be connected to an A/D converter, allowing the process control computer to determine the temperature of the process and to thereby make adjustments to the process in real-time.
One important characteristic of an A/D converter is the converter's dynamic range. The dynamic range is a measure of the magnitude range over which the input to the converter may vary.
In the process control application example given above, the temperature may vary, for example, between 0.degree. C. and 500.degree. C., with the corresponding output of the thermocouple varying between zero and one volt. Thus it can be seen that the A/D converter must be capable of resolving the possible one volt variation of the thermocouple input signal into a given number of bits, the number of bits being determined by the resolution of the A/D converter. A number of commercially available A/D converters readily provide eight bit resolution, thus the one volt input may be resolved into 256 discrete values. Hence the 0.degree. C. to 50.degree. C. range of temperatures could be determined to an accuracy of approximately 0.2.degree. C.
A problem arises, however, when the dynamic range of the A/D converter must be increased. For example, in the previous application if it were necessary to determine the temperature over a range of 0.degree. C. to 100.degree. C., the output of the thermocouple may vary between zero and two volts. Inasmuch as the A/D converter may only be capable of accepting a maximum input signal of one volt, the dynamic range of the A/D converter would be exceeded. Previous approaches to utilizing a lower dynamic range converter with a higher dynamic range input signal have generally been of two types. One approach has been to provide an A/D converter having a non-linear transfer function. Thus a smaller number of output codes are required to represent the possibly large variation in the input signal. This approach however, has the disadvantages of increased circuit complexity, such as requiring non-uniform resistance ladder networks, and of a non-linear output which complicates subsequent processing by a digital computer.
Another approach to providing an A/D converter having a large dynamic range is to place a scaling amplifier ahead of the A/D converter, the purpose of the scaling amplifier to divide or compress the large dynamic range into various linear ranges, each of which has a lesser dynamic range. While this technique may crudely approximate the desired performance, it also has the disadvantage of increasing circuit complexity, component count, and power consumption.
A further problem is encountered when it is necessary to operate the A/D converter at a high conversion rate, such as when the analog signals from a large number of sources must each be scanned and digitized within a small interval of time. One such application requiring high speed conversion is the scanning and digitizing of an array of photodetectors. For example, in infrared detectors characterized by large focal plane array detectors the dynamic range of the output signal may be 2.sup.14 and higher, while the number of elements to be scanned may be several hundred or even a thousand. In this application, however, the noise associated with a signal towards the top of the dynamic range is typically many times greater than that of a signal near the bottom of the dynamic range. Properly designed compression takes advantage of this.
In order to operate at the speeds required to digitize the analog signals from a large number of sources in a short period of time, it is often necessary to utilize a flash A/D converter. However, a problem arises in that current commercially available flash A/D converters are typically limited to eight bits of resolution, which is incompatible with the 2.sup.14 dynamic range of the analog signal from the infrared detector.
In order to overcome these limitations it has been known to use several flash A/D converters in a successive approximation scheme, or to use range selection amplifiers at the front end of the A/D converter, or to use both schemes in one design. A problem arises, however, especially when the number of channels of data increases to several hundred as in the aforementioned large focal plane array detector application, in that the additional circuitry significantly increases the size, weight and power consumption of the A/D converter circuitry. This problem is especially acute in airborne applications, as when the infrared detector is carried aboard a spacecraft or an aircraft.
It may be appreciated that if a large number of data channels must be processed in a short interval of time, as when a focal plane array serves as an imager in a real-time imaging application, an additional amount of computer processing is required to convert the compressed non-linear readings back into a linear format. Such additional processing may place substantial burdens on the computer hardware and software, such as requiring faster, more complex computer components.